The invention relates to a process for automatically screw-connecting two components, and to a stud and a union nut for carrying out the process. Preferred embodiments of the invention relate to such processes and devices with which an automatic fitting machine utilizing a torque controlled tightening method can be used.
"Die Flexibilisierung der Schraubmontage" [Screwed fitting made versatile] by A. Stapel, published in "Technische Rundschau" 39/85, discloses a process for automatic screwed fitting. The process is carried out, with the aid of an automatic fitting machine, in two steps, in that the components which are to be connected first of all are screw-connected until a predetermined preliminary tightening moment has been reached and then are tightened until a final torque value has been reached. The tightening moment has to be monitored permanently throughout the two fitting steps and the tightening operation has to be interrupted when the respectively predetermined limit values have been reached. If the two components are to be connected via a plurality of screws, it is necessary for the tightening moments of each individual screw to be monitored, this rendering the technical outlay for monitoring purposes very high. Irregularities in the surfaces of the components or settling may result in the screws each being located in different angular positions following a fitting step. This makes it difficult for the fitting tool to be attached easily and quickly at the beginning of the new fitting step or for the purpose of releasing the screw-connections. Another disadvantage is that, when conventional screws are used for transmitting high screw forces, it is necessary to apply a large tightening moment, which, in addition to corresponding dimensioning of the automatic fitting machine, also presupposes the use of suitable screws, which are of stable enough design to absorb high torsional and longitudinal forces.
An object of the invention is to specify a process, as well as a stud and a union nut for use in this process, for automatically screw-connecting two components using the torque-controlled tightening method, by means of which it is possible for the screw-connection operation to be executed quickly and a secure connection between the two components is obtained.
This object is achieved according to preferred embodiments of the invention, utilizing a process for automatically screw-connecting first and second components by a torque-controlled tightening method with an automatic fitting machine and a connecting element which screw-connects the components, wherein the connecting element has two same-direction threaded sections of different pitches and each threaded section is screw-connected to one component in each case, with the following process steps for completing the fitting operation: rotary-angle-controlled preliminary tightening of the connecting element, termination of the rotary-angle-controlled preliminary tightening when there is contact between mutually facing surfaces of the components which are to be connected, and then torque-controlled definitive tightening until a predetermined final torque value has been reached.
This object is also achieved according to preferred embodiments by utilizing a stud which has angular markings on the screw head indicating the angular position thereof.
This object is also achieved according to preferred embodiments by utilizing a union nut with a first internal thread extending no more than approximately one-half the length of the nut, which threaded section is adjoined by a non-threaded section.
The rotary-angle-controlled preliminary tightening of the screw during the first fitting step can take place at a considerably higher angular velocity than the hitherto conventional torque-controlled preliminary tightening since permanent torque monitoring is dispensed with. In the case of rotary-angle-controlled preliminary tightening, the screw is rotated by a certain angle, which can be set in the automatic fitting machine; in contrast to the torque-controlled process, it is not necessary to monitor the connection throughout the entire tightening operation, with the result that the first fitting step can be carried out swiftly and with low outlay, without torque monitoring.
During preliminary tightening, the two components which are to be connected advance towards one another, the mutually facing surfaces of the components which are to be connected expediently coming into contact with one another once the end position of the first fitting step has been reached. The use of a connecting element with two same-direction threaded sections of different pitches means that the components advance towards one another slowly, the advancement speed depending on the difference between the thread pitches of the two threaded sections. It is possible here for very coarse threads to be used, since it is only the difference between the respective thread pitches which is a measure of the advancement speed of the components. Thus, just a small difference between the thread pitches here means that the components advance towards one another at a low speed during the screw-connection operation. Such connecting elements with coarse threads are cost-effective to produce and are capable of absorbing high forces.
The low advancement speed reduces the risk of the components being pressed against one another with undesirably high force once the predetermined final angle value has been reached. As a result of the small amount of axial displacement per screw revolution, slight deviations, for example, in the surface characteristics of the components do not result in the components being destroyed inadvertently. The component contact surfaces, which butt against one another following the first fitting step, are only pressed against one another by a small torque, which, in the second fitting step, is usually exceeded in any case once the predetermined final torque value has been reached, with the result that the risk of the connection being inadvertently tightened too much is reduced.
Following termination of the first fitting step, the connecting element is located in a very specific angular position, which is known to the automatic fitting machine, with the result that, for example once there has been a change of tools, the fitting tool can easily engage in the wrench attachments of the connecting element.
In the second fitting step, torque-controlled definitive tightening of the connection takes place until the predetermined final torque value has been reached. This ensures that the connection has a retaining force which is adapted to the respective requirements. Thus, damage to materials, for example as a result of over-expansion, is avoided. The use of the connecting element with two same-direction threaded sections makes it possible to achieve a high screw force with just a low screw tightening moment.
The process is advantageously suitable when using at least two connecting elements, the longitudinal axes of which are preferably aligned parallel to one another. It is possible for a plurality of connecting elements to be tightened simultaneously by one automatic fitting machine. Following termination of the first fitting step, all the studs assume the same angular position, this rendering renewed attachment of the fitting tool easier.
Prior to the rotary-angle-controlled preliminary tightening, the stud first of all is expediently pre-fitted in one of the components and then, for further screw-connection, is brought, together with the first component, into alignment with the bore of the second component. This preliminary step means that the operation of tightening the screw is separate from that of positioning the components. The studs can be pre-fitted in the first component in a position which is favorable for fitting purposes, the stud, in this pre-fitted position, being retained in captive fashion in the first component.
Both a stud and a union nut are suitable as the connecting element for carrying out the process.
The stud has a marking which indicates the angular position of the stud. This marking can be provided by the shape of the screw head, which is preferably designed symmetrically with respect to at most one longitudinal plane of the stud. According to a further refinement, it is also contemplated for the screw head to be of completely non-symmetrical design. This permits immediate recognition of the angular position of the stud both by optical and by sensory means, with the result that the fitting tool can automatically be brought into the engagement position with the screw head.
In each case one screw head may be provided on the two end sides of the stud, in order to make it possible, depending on the given design, for the studs to be screwed into the component from different sides.
The beginning of the thread and the end of the thread of a threaded section are expediently located in a common longitudinal plane of the stud. In this respect, the thread itself is also aligned in an angular position which can be recognized by the automatic fitting machine, which makes it possible for the threaded section to be screwed into the respective components, and unscrewed therefrom, with precision.
The union nut which is suitable for carrying out the process has a first threaded section, which is designed as an internal thread, extends over not more than approximately half the length of the union nut and is adjoined by a thread-free inner section. A first embodiment provides that the second, same-direction threaded section of different pitch is also designed as an internal thread, the thread-free section being arranged between the two internal threads. Another embodiment provides that the second threaded section is designed as an external thread on the outside of the union nut. In this case, one section of the union nut is designed as a nut head, which advantageously has a larger diameter than the external thread.
The union nut is suitable for screw-connecting two threaded stubs.